Capacitive sensing for degradation assessment in bismaleimide/glass-fiber composites

New aircraft designs increasingly contain more components made from polymer matrix composites, in both primary and secondary roles. This demands new monitoring techniques that emphasize the role played by polymer matrix degradation in the failure of these parts. The polymer matrix does not take significant load in tension but must transfer shear load. The present work investigates changes in surface chemistry due to accelerated thermo-oxidative aging on a bismaleimide (BMI) laminate system reinforced with woven glass fiber. BMI is used for high temperature applications up to 230 °C. Samples are isothermally aged at various temperatures whose selection is guided by degradation steps revealed by thermo-gravimetric analysis. Permittivity of aged samples is measured as a function of frequency from 200 Hz to 2 MHz, using a parallel-plate electrode design. Results are compared with measured interlaminar shear stress to assess the suitability of capacitive sensing as a method for nondestructive monitoring of thermo-oxidative aging in this material system.